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Full text of "Handbook Of Chemical Engineering - I"

344
CHEMICAL ENGINEERING
pass through, the surrounding unsaturated solution. Therefore to increase the speed of dissolution we must decrease the size of the film (and especially in cases where the speed of solution is chiefly dependent upon the speed of diffusion as in the case quoted above) by setting up a flow of solution past the solid particles. Few experiments have been made in this connection due to the difficulties encountered in getting so
« 30
10
0 10
0.40
0.20              0.30
Time Required FIG. 3.—Effect of agitation on solution.
any exact data.    However if L be the thickness of the concentrated film around the particles of soluble material we get from our first expression
dx/dt = KS (a - z) or
I It loge a/a - x = KS/L                                          (3)
and since L will depend upon the number of revolutions N of the stirrer used to agitate the mixture we should by experiment be able to find a relation between L and N. In an experiment on dissolving benzoic acid in water it has been found that l/t log* a/a — x = BN% where B is a constant
is a constant in any one case we find
rp   __   ____y_
1  " BNH
(4)
By substituting different values of N we then get the curve shown in Fig. I.1 The power consumed by an agitator varies with the speed of agitation and Fig. 3 is a curve derived from experimental work to determine the relation between dissolution and speed of agitation. From this we see consideration must be given to the cost of power as against agitation capacity as decreased time in agitation means greatly increased power consumption.
Agitation is necessary so long as there is any soluble solid present due to the fact that the film is continually being reformed.    However, when all the soluble matter
1 The values of time obtained here are valuable only in so far as they give us a graphical illustration of the effect of increasing the velocity of the solvent past the particles. A much more valuable experiment would be to suspend the particles by means of threads or wires in the cross-section of a tube and vary the flow of the solution so that there would be no question as to the efficiency of the stirrer in forcing the solution to move past the particles.